1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a copy machine or a facsimile machine.
2. Description of the Related Art
Generally, in an image forming apparatus, after electrostatically charging an outer peripheral surface of a photosensitive drum serving as an image bearing member, in a uniform manner, the photosensitive drum is exposed to light according to image data to form an electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is developed with toner into a toner image, and the toner image formed on the photosensitive drum is transferred onto a recording sheet by a transfer roller. The recording sheet is then transported to a fixing unit, where the toner image is fixed onto the recording sheet.
The photosensitive drum includes a so-called “OPC photosensitive drum” formed by coating a metal pipe with an organic material, and a silicon photosensitive drum wherein amorphous silicon is vapor-deposited on the outer peripheral surface thereof. The silicon photosensitive drum has high surface hardness, excellent wear resistance and high durability.
Although excellent wear resistance is an advantage of the silicon photosensitive drum, it also has a negative side. For example, the silicon photosensitive drum involves a problem about image deletion occurring under high-temperature/high-humidity environments to cause fogging or blurring of images. The image deletion is attributed to a phenomenon that a charging product (i.e., a product generated during the charging process) accumulated on the outer peripheral surface of the photosensitive drum absorbs water under a high-humidity environment and disarranges an electrostatic latent image. Thus, it is necessary to abrasively remove an oxide consisting of the charging product accumulated on the outer peripheral surface of the photosensitive drum, so as to prevent occurrence of the image deletion.
Typically, a sponge roller is used as a means to polish the outer peripheral surface of the photosensitive drum. The sponge roller is disposed to be in press contact with the surface of the photosensitive drum on an upstream side relative to a cleaning blade, and adapted to be rotated at a peripheral speed having a certain difference from that of the outer peripheral surface of the photosensitive drum so as to perform polishing using toner including abrasive particles. The toner comprises a binder resin serving as matrix particles thereof, and an abrasive, such as silica, alumina, zirconia or titania, attached on surfaces of the matrix particles. The toner is used for forming an image by itself, and a remaining part of the toner untransferred in a transfer unit is also used for polishing the outer peripheral surface of the photosensitive drum.
There has been known a technique of performing the polishing using a cleaning unit 100 which comprises a polishing roller 101 and a cleaning blade 102, wherein the polishing roller 101 is arranged to be located below the cleaning blade 102, as shown in FIG. 6 (see, for example, JP 2004-361775A). In this arrangement, when the polishing roller 101 is rotated in a direction reverse to that of a photosensitive drum 110, a toner pool B is formed around a nip zone between the polishing roller 101 and the photosensitive drum 110 (specifically, the nip zone and an outlet area thereof) to hold toner having an abrasive attached thereon, so that an outer peripheral surface of the photosensitive drum 110 can be effectively polished. However, if the toner is excessively accumulated in the toner holding space B even though a part of the toner in the toner pool B is attached onto an outer peripheral surface of the polishing roller 101 along with the rotation of the polishing roller 101 and scraped off by a scraper 103, another toner pool C will be formed around the polishing roller 101, e.g., on a bottom of the cleaning unit 100. This is likely to cause a problem that accumulated toner in the toner pool C spills out of the cleaning unit 100 and falls onto a recording sheet to stain an image thereon.
As one of the measures for preventing such an image stain due to falling of toner, it is contemplated that the cleaning unit 100 illustrated in FIG. 6 is turned upside down to allow the polishing roller 101 to be located above the cleaning blade 102. However, in the arrangement where the cleaning unit is turned upside down to allow the polishing roller to be located above the cleaning blade, it is assumed that the surface of the photosensitive drum cannot be adequately polished. Specifically, if the polishing roller is designed to be rotated in a reverse direction to a rotation direction of the photosensitive drum, a toner pool cannot be formed around an outlet area of the nip zone between the polishing roller and the photosensitive drum to preclude effective polishing. Conversely, if the polishing roller is designed to be rotated in the same direction as the rotation direction of the photosensitive drum, undesirable jitter will be caused by rotational fluctuation of the photosensitive drum, although polishing is effectively performed based on strong stress between the polishing roller and the photosensitive drum.